Updated at: 25-07-2022 - By: Lucas

An internal combustion engine would be incomplete without a spark plug. This can be seen in several ways, including the colors on the spark plugs that indicate specific problems with the plugs, but these colors can also suggest a potential engine problem.

Whether black, white, red, brown or yellow, each of these colored deposits has a certain meaning behind it.

On the insulator of spark plugs, brown and yellow deposits can accumulate. In this post, we’ll look at some of the potential causes of the formation of these colorful deposits.

The formation of black and white deposits on spark plugs has been the subject of previous articles in this series. You may want to take a look at those posts.

Diagnostics are the only way to determine the cause of brown and yellow deposits on the igniter of the plug. Visual inspections are rarely helpful in solving the problem, but car owners can occasionally readily cope with this chore.

What are the causes of brow deposits on spark plugs?

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Long-term use of spark plugs causes brown deposits to accumulate. Not only does this color appear odd, but it may also indicate that something is wrong.

Low-quality fuel

Most automotive experts believe that using low-quality fuel is a common cause of brown deposits on spark plugs.

After a while, looking at the spark plug insulator will not reveal any fuel-related difficulties. Only after a thorough examination of the plug will it be possible to determine the specific cause of the brown spots on the plug.

Fuel is leaking into the spark plug due to a malfunctioning injector, which is a result of low-quality fuel blockages.

Because of this, a rusty brown deposit forms on the spark plug insulator, the electrode is unable to burn out the excessive mixture, and some of the mixture soaks into the igniter’s metal body.

Other causes

The hue of a rusty brownish insulator can vary greatly. Based on this, you may pinpoint the exact location of the problem in the vehicle. Because of an air filter blockage, more fuel mixture was able to get into the combustion chamber and burn.

Oily liquid can leak into the combustion chamber if the piston rings are damaged, and this oily liquid can leave scars on the insulator over time.

Caps may not be suitable for connecting the mass to the igniters, and this is not ruled out. These parts need to be replaced at least once a year.

Petrol with a reddish brown tint has been injected with iron and carbon compounds. Poor fuel quality and iron-containing subsidence are related with all hues of red regardless of the tint or other colors they are mixed with.

Why do yellow deposits form on spark plugs?

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The yellow deposits on the spark plugs are another problem. In addition to using low-quality fuel with additives, the most typical reason for this is the development of these yellowish deposits.

A higher concentration of the fuel’s harmful additives can be found in the final mixture. While it isn’t dangerous right away, it has the potential to permanently harm the engine if left unchecked.

Gas leaks can be identified by a yellowish deposit on the spark plug. Insulation gets yellow and there is a black patch between metal and ceramic as a result of this.

You can determine whether your spark plugs need to be replaced by the yellow deposits they leave behind. Because of the yellowish buildups on spark plugs, drivers often have trouble starting their engines.

Yellow deposits on spark plugs can also be caused by:

  • Problems with some individual car parts;
  • Poor engine efficiency.

It’s a good idea to check out the cylinder’s valves and divider if you notice yellow stains on your spar plug electrode.

Drops of oil and metal fragments on the electrode often accompany such incidents. The engine will begin to lose power as soon as it is started.

On ceramics, In addition to upgrading the fuel, you should consider how often the caps used to replenish the igniters need to be replaced.

Over time, these parts harden to an extreme degree and lose their ability to cling to the ceramic body, resulting in a failure to start the engine.

There may be a discharge on spark plugs that causes the yellow coloration to appear. That’s why it’s best to replace the engine’s protected wires. The discharge can be seen by opening the hood and starting the engine in the dark; the blue glow around the spark plugs will be visible.

A visual inspection, like in the instance of brown deposits, will not assist in resolving the issue; only a proper diagnosis performed by a licensed repair facility can.

If you notice any abnormalities with the engine’s normal operation, such as a hard or bad start, you should investigate the cause immediately.


Spark plug deposits often point to a problem with the plugs themselves, but they can also point to issues with other components of the engine. Because it’s difficult to figure out what’s wrong, you’ll want to take it to a reputable repair shop.